1. Field of the Invention
The present invention relates to a beam writer that forms a circuit pattern on a substrate such as a film board and a glass board, or that forms an image on a sheet for recording. Especially, it relates to controlling beam-modulation.
2. Description of the Related Art
A beam writer, such as laser writer, forms an electric circuit pattern on a substrate, such as a silicon wafer, and so on. While, a laser scanner forms an image on a paper. The above beam writer has, for example, an exposure optical system with a polygon mirror and an f-θ lens, and scans an electronic beam or laser beam on a photosensitive material by using the exposure optical system. Then, an optical-modulator incorporated in the exposure optical system, such as an AOM (Acousto-Optical Modulator), modulates the beam. Namely, the optical modulator controls the exposure on the photosensitive material by passing and intercepting the beam in accordance with pattern-data. The beam modulation is controlled by a sequence of high frequency control pulse signals.
The scanning-speed of the beam is not always constant due to the precision of the f-θ lens and the polygon mirror. The differing precision results in an irregular scanning-speed, in other words, the irregular scanning-speed is caused by degrees of the precision. To correct the irregular scanning-speed, the frequency of the control pulse signals for modulating the beam is modified in accordance with the variation of the scanning-speed. For example, a scanning line is divided into constant distance-intervals, and a time-interval, which is a pass-time of the beam in each distance-interval, is successively measured along the scanning-line before forming the pattern. Then, when scanning the beam, the clock frequency is adjusted in accordance with the detected series of time-intervals. In order to minimize a dot-pitch of the pattern, as much as possible, a PLL (Phase-Locked Loop) circuit is utilized to generate clock pulses with an accurate frequency.